Updated: May 10, 2020
"Ethereum is a decentralized platform that runs smart contracts: applications that run exactly as programmed without any possibility of downtime, censorship, fraud or third party interference." - https://www.ethereum.org
The 'fuel' that runs the system is called 'Ether', which has many parallels to the more well known 'Bitcoin'. It is an exciting new prospect in crypto-currency and is poised to rapidly increase in value. It's fundamental structure has more real-world applications than Bitcoin, and therefore has a much higher potential commercial value.
Predction: Ether will surpass Bitcoin in value.